He is saying that his opinion may be bad but he is on the same side as Cates and Rachel.
He is doing this by using a comparison between their situation and bread and butter. His opinion is viewed as bad so therefore his opinion is "rancid butter". A piece of bread, like everything else, has two sides. The two sides in this case are for evolution in the classroom and against evolution teachings in the classroom. Naturally, you put butter on bread. Hornbeck is saying that people may view his opinion as wrong but he a supporter of what they believe therefore "the butter is on their side of the piece of bread".
He is doing this by using a comparison between their situation and bread and butter. His opinion is viewed as bad so therefore his opinion is "rancid butter". A piece of bread, like everything else, has two sides. The two sides in this case are for evolution in the classroom and against evolution teachings in the classroom. Naturally, you put butter on bread. Hornbeck is saying that people may view his opinion as wrong but he a supporter of what they believe therefore "the butter is on their side of the piece of bread".